Selecting a sunn hemp cover crop genotype for weed suppression and seed production

Project Overview

Project Type: Research and Education
Funds awarded in 2008: $170,000.00
Projected End Date: 12/31/2011
Region: Southern
State: Florida
Principal Investigator:
Dr. Carlene Chase
University of Florida

Annual Reports


  • Animals: bees


  • Crop Production: cover crops
  • Education and Training: focus group, on-farm/ranch research
  • Farm Business Management: feasibility study
  • Pest Management: allelopathy, cultural control, weed ecology
  • Production Systems: organic agriculture
  • Soil Management: green manures, nutrient mineralization, soil chemistry
  • Sustainable Communities: sustainability measures


    Studies were conducted in 2008 and 2009 to evaluate the potential for sunn hemp as both a cover crop and as a seed crop in Griffin (Georgia), Gainesville (Florida), and Lajas (Puerto Rico). Field trials were conducted to evaluate the phenotypes of sixteen accessions of sunn hemp. In Georgia, significant differences were found among the sunn hemp accessions tested, for all morphological traits evaluated at 2 and 4 MAS. Principal component analysis showed that the first principal component accounted for 77% of the total variation while principal components 2 and 3 progressively accounted for 87 and 92%, respectively of the variation and average linkage cluster analysis grouped the original 16 accessions into well-defined phenotypes with 4 distinct groups and one outlier based on total biomass which included totaled morphological data and number of seeds produced. In Florida the 16 accessions could be separated into two distinct groups based on several vegetative and reproductive parameters. Accessions in Group 1 were tall with few branches, and produced few to no flowers and pods. These accessions included 2 (PI 234771), 3 (PI 248491), 7 (PI 295851), 14 (PI 468956), 15 (PI 561720), and 16 (PI 652939). Accessions in Group 2 flowered early (49-66 days after seeding) and seemed to be daylength insensitive. These accessions were shorter in stature, and lower in biomass than those in Group 1, but produced more branches, flowers, and seed. These included accessions 1 (PI 207657), 4 (PI 250485), 5 (PI 250486), 6 (PI 250487), 8 (PI 314239), 9 (PI 322377), 11 (PI346297), 12 (PI391567), and 13 (PI426626). In Puerto Rico, the accessions 2 (PI 234771), 12 (PI 391567), and 16 (PI 652939) were found to have potential for biomass and/or seed production.

    A second field study was conducted in 2008 and 2009 to investigate the effects of seeding rates (10, 25 and 40 lb/ac) and removal of apical dominance on weed suppression and seed production. In Georgia, similar weed biomass amounts were detected regardless of sunn hemp cutting date both years and a lower seeding rate for sunn hemp as a cover crop for weed reduction is feasible. Additionally, these results show a sunn hemp grower that low seeding rates are as effective as higher rates. In Gainesville, cutting to break apical dominance had no significant effect on weed suppression and on flowering, but did induce branch formation. All three seeding rates had lower total weed biomass, but were not significantly different from one another. No seed production occurred in Florida and Georgia. In Puerto Rico, the highest biomass and seed yields were obtained with the intermediate planting density of 25 lb/ac, and apical cuttings did not improve C. juncea yields or weed suppression.

    Phytotoxicity was observed with aqueous extracts of all fourteen accessions screened with a bioassay for allelopathic potential. High performance liquid chromatography provided evidence of a compound in foliar extracts with characteristics of 5-hydroxynorleucine, the amino acid derivative with allelochemical properties previously reported only in sunn hemp seeds. Sunn hemp also was shown to have the potential to serve as a catch crop for potassium. However, residue from sunn hemp seed crops was recalcitrant showing negligible decomposition over a 12-week period.

    A study was also conducted to evaluate the economics of utilizing sunn hemp as a cover crop. Partial budgets were prepared for five summer fallow treatments; sunn hemp, velvet bean, cowpea, sorghum sudangrass, and tillage. These treatments are compared for weed suppression, nitrogen contribution, and the potential impacts on a following cash crop of squash. Sunn hemp was the least expensive summer fallow treatment, followed by velvet bean, cowpea, sorghum sudangrass, and tillage.

    Project objectives:

    (1) Evaluate the effect of different geographical locations on biomass accumulation, flowering and seed yield of the USDA’s sunn hemp germplasm collection;

    (2) Identify bee species visiting sunn hemp flowers to determine which are effective pollinators and quantify their visits;

    (3) Assess the phenotypic variability of flowering and characterize the sensitivity to environmental factors;

    (4) Investigate the effects of breaking apical dominance on weed suppression and seed yield, and compare the allelopathic potential of the accessions;

    (5) Determine how cultural practices for sunn hemp seed production influence nitrogen accumulation, decomposition, and plant available soil nitrogen; and

    (6) Evaluate the economic costs and benefits of sunn hemp domestic seed, cover, and fodder crops.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.